DWIGHT W. i - 
THE TANGANYIKA PROBLEM. 143 
have had a different origin from the general African fresh- 
water constituents, it is highly probable that some of the 
fishes which we now, from their dispersal, regard as belong- 
ing to the normal African fresh-water series, are in reality 
scattered members of this same halolimnic group. At all 
events it is quite clear that it is not correct to suppose 
that the fish-fauna of Tanganyika is at all similar to that 
of the other African lakes, and this strikingly distinctive 
character of the Tanganyika fishes constitutes the primd 
facie evidence for supposing that at least some of the fishes 
which characterise the fauna of Lake Tanganyika are of 
the same stock as the halolimnic invertebrates themselves. 
The halolimnic fauna consists then of a group of animals, 
the invertebrate section of which is rigidly confined to the 
lake, and which have no obvious relation to the normal 
African fresh-water invertebrates, but it is also indicated that 
some of the African fishes which we have hitherto regarded 
as normal fresh-water fishes of the African continent may 
belong in reality to the now scattered vertebrate section of 
the halolimnic group. 
It remains for us now then to examine in much greater 
detail the characters of the individual components of the 
halolimnic group, and thereby to attempt to acquire the 
information necessary in order to ascertain the actual 
affinities of these forms ; while finally we shall have to 
consider the question of the nature and the origin of the 
whole of the halolimnic group in Tanganyika, and this 
question itself forms the Tanganyika problem as it now 
exists. But before proceeding to discuss in detail the 
structure of the halolimnic forms, it is desirable to con- 
sider certain widespread phenomena relating to the normal 
fresh-water fauna of the African lakes in general. 
O 
